Jack-up drilling unit with tension legs

ABSTRACT

The invention relates generally to a method for supporting non-buoyant offshore drilling units that are anchored to the sea floor and are subject to heavy lateral loads. Specifically, tension tendons are attached to piled foundations to prevent jack-ups from being overturned by the loads. Tension can also be applied to the tendons to provide additional hold down forces to the offshore units.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims benefitunder 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/754,856filed Jan. 21, 2013, entitled “JACK UP DRILLING UNIT WITH TENSION LEGS,”which is incorporated herein in its entirety.

FEDERALLY SPONSORED RESEARCH STATEMENT

Not Applicable

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

GENERAL FIELD

The disclosure relates generally to offshore drilling units that areanchored to the sea floor, and particularly to jack-up rigs.

BACKGROUND

The oil and gas industry has been expanding its exploration andproduction operations from land to sea since the 1890s. The firstsubmerged oil well was drilled in fresh waters in Ohio in 1891. In 1897,the first derrick was placed atop a wharf about 250 feet from theCalifornia shoreline. However, true offshore drilling and production didnot take off until the first well was drilled completely out of site ofland in 1947. Since then, advancing technology has allowed for thedrilling of wells and recovery of oil and gas at greater water depths.However, difficulties still remain, especially in waters known for largewaves, moving sea ice or icebergs, and landfast ice.

FIG. 1 displays commonly used offshore drilling units. Some units, suchas the semi-submersible and drilling ship are moored to the sea floor tominimize lateral and vertical displacement. Such mooring systems caninclude catenary mooring, where the lines are loose, andbuoyancy-corrected weight of the suspended part of the mooring line mustbalance the station-keeping moment in the catenary mooring system.Another option is a taut mooring system, where the lines are light,tightened to near straightness, and axial elasticity accommodates wavemotion. Semi-taut systems are also available.

Others, such as the platform rig, are permanently anchored to the seafloor. Of particular interest is the jack-up rig because it is a mobileoffshore drilling unit that anchors to the sea floor and is raised abovethe water for use.

The jack-up is designed to be towed (though some may be self-propelled)to the drilling site and be ‘jacked up’ above the water so that the waveaction of the sea only impacts the legs, and not the main body of thedecking. Because the legs generally have a smaller cross section, thewave action can pass by without significantly moving the JU. However,jack-up rigs are limited to water depth of less than 400 feet due to thelimits on leg length.

Generally, jack-up platforms are triangular in shape with three legsthat bear the weight of the unit equally. However, the platform shapeand number of legs can change to accommodate a larger unit. Furthermore,during a ‘preloading’ phase, ballast water and the platform are used toweigh down the legs and drive them into the sea floor such that theycannot penetrate further during operations, thus shifting the deck whilein use.

Jack-up units typically include the drilling, production, and/orworkover equipment, a leg jacking system (typically a rack and pinionsystem), crew quarters, loading and unloading facilities, storage areasfor bulk and liquid materials, helicopter landing deck and other relatedfacilities and equipment.

The advantage of the jack-up rig is that as a floating platform it canbe moved easily from site to site. However, many jack-ups are nowoperating for extended periods at one location, especially those actingas production units. Furthermore, many jack-up rigs are self-elevating.A rack and pinion system located on the platform is used to lower thelegs to the sea floor when they continued to be ‘jacked’ until the hullis raised above the water. By having the ‘jacking’ system on board, therig can quickly be moved in case of an emergency.

However, jack-up rigs do have some disadvantages. Jack-up rigs aresubject to a number of internal and external loads, which when combined,can lead to bending, compression, buckling, fatigue or overturning. Assuch, jack-up rigs have limited resistance to external loads, which is afunction of the jack-up design and available downward force, limited bythe weight and loads on the jack-up. The horizontal environmental loads,due to wave, wind, current, and ice, as applied to the deck and legs,especially when the unit is ‘jacked up,’ can generate enough force tooverturn the structure. Ice loads in particular can be very highexceeding wave and winds loads by a very high factor and cause a jack-upto slide.

For instance, jack-up rigs are finding increasing use in offshore Arcticdrilling, especially in more shallow waters. However, the Arctic is aremote and harsh location where ice on the water creates considerablechallenges to prospecting for and producing hydrocarbons. The ice floesfound in the Arctic can collide with the rig legs and cause structuraldamage to the legs or push the rig off its mooring. The horizontalloads, combined with the internal compression loads caused by needing alarger platform, more equipment, and ice weight on the jack-up itself,can also facilitate buckling of the rig. This is problematic becauseeven small shifts can result in a hydrocarbon leak. Any risk of such aleak is completely unacceptable in the oil and gas industry, to theregulators and to the public, particularly in pristine Arcticenvironments.

Many inventions have been made to increase the stability of jack-uprigs, especially in the Arctic. For instance, US20120125688 proposes anice-worthy jack-up (IWJU) wherein the hull includes an ice bending shapethat starts near the deck and extends downwardly to the bottom of thehull. The hull is lowered into the water into an “ice defensiveconfiguration” in the absence of ice. Thus, when ice moves towards theIWJU or when there is landfast ice, the legs of the IWJU are protectedby the ice bending hull. While this protects the IWJU from structuraldamage, it does not address the issue of overturning.

Even in milder conditions, jack-up rigs face threats of being overturnedby currents, waves, and wind. U.S. Pat. No. 4,265,568 discloses ajack-up rig with a single leg attached to a polygonal-shaped gravitybase that is capable of penetrating the sea floor. The gravity baseprovides some protection against overturning forces. Other designs wellknow in the oil and gas field include attaching the legs to spud cans ormats anchored onto the sea floor.

What is needed in the art is a better method of supporting offshoredrilling units anchored to the sea floor, such as the jack-up, againstheavy wind, wave or ice loads.

Furthermore, what is needed in the art is a method of supportingjack-ups against horizontal loads wherein the support system can bereleased quickly.

SUMMARY

The present disclosure relates to a method of protecting offshoredrilling units that are anchored to the sea floor, such as a jack-up(JU), from being overturned by horizontal loads. Specifically, tendonsare attached to the sea floor anchoring foundation and to the rig toprevent the unit from overturning from heavy horizontal environmentalloads.

U.S. Pat. No. 4,604,001 describes a tension-leg platform that usesjack-up legs to secure the platform while tendons are attached andtightened. However, in use this platform is submerged to a depth belowthe level of the majority of the surface forces, such as, for example,150 to 200 feet. Thus, the design and specifications are quite differentfrom a jack-up rig. Further, the legs are only used during placement ofthe tendons, and then jacked-up out of the way again.

The devices and methods of the present disclosure is similar in somerespects, however, to the tension-leg platforms (TLP) (see e.g., FIG.1B-C). The TLP is a compliant structure that responds dynamically towave excitation. In a free floating condition, the platform would behavelike a semi-submersible and move in the six degrees of freedom that areassociated with all floating bodies; surge, sway, heave, roll, pitch andyaw. The tension legs resist heave, pitch and roll, but surge, sway andyaw still occur and are dependent on wave excitation and the responsecharacteristics of the platform.

In the present disclosure, the tendons are used to offset horizontalloads that may overturn the anchored offshore unit, thus providingadditional support. This is done by tensioning the tendon, which placesthe units leg in compression. Thus, the combination of jack-up legstogether with tension tendons provides resistance to horizontal movementand overturning.

The tension tendons (aka tension legs) can be used with any type ofjack-up leg, including but not limited to open-truss legs and columnarlegs. Furthermore, the tension tendons are independent and in additionto other stabilization systems, including the legs and anchoringfoundations such as mats and spud cans.

While it is possible to integrate the tendons into the jack-up legs,this may not be a preferred configuration because the tendons may be tooheavy for the leg structure, especially where tubular joints are usedfor the tension legs. Also, external tendons may be advantageous becausethey will be easier to replace. Thus, attachment points are required toreversibly couple or attach the tendons to both the offshore unit and tosome foundation structure on the sea floor. Such attachment points canbe any place convenient, but should be attached to something configuredto handle the load, and include the leg structures and/or the hull.

Top connectors for TLPs, include those described in US2009290939 andwhich permit some rotational movement of the connector, although it isexpected that little rotational movement need be accommodated hereinsince the rig also has solid legs. In the connector of US2009290939, thetension leg buoy has a bowl-shaped extension that transfers the tensionof the tendon to the flex connector. A dome-shaped structure at leastpartially surrounds the bowl-shaped extension, but is spaced apart fromit to permit at least some rotational movement of the connector. Thedome-shaped structure may be attached by clamps or other mounting meansto a flange member affixed to the tendon porch. This arrangementprovides a load path when the tendon top connector is reverse loadedthat extends from the tendon length adjustment joint, through slips,through the bowl-shaped extension, through the dome-shaped structure,through the segmented clamps, and finally into a flange attached to thetendon porch. In this way, detachment of the tendon top connector isprevented if a reverse load is applied such as may occur during extremeocean conditions.

U.S. Pat. No. 4,871,282 describes another top connector. In this patent,the upper connector for each tendon includes a housing with a conicalshoulder located therein. A terminal segment on the upper end of eachtendon extends through the housing. Dogs are carried on the shoulder ofthe housing, each having threads on the interior for mating threadsformed on the terminal segment. A cam ring moves the dogs from an outerretracted position to an inner engaged position. The cam ring also willrotate the dogs relative to the terminal to mesh the threads of the dogswith the threads of the terminal segment.

U.S. Pat. No. 5,020,942 describes a TLP top connector that has a housingwith a bore containing a conical shoulder. Several segments locate onthe conical shoulder and slide between an upper retracted position to anengaged position. In the engaged position, threads on the interior ofthe segments engage threads formed on the exterior of the tendon. A camplate slides the segments down when the cam plate is rotated. A guidering mounts outward of the segments. The guide ring has fingers thatengage slots in the backs of each of the segments, and the fingers andvertical slots allow the segments to move axially, but prevent them fromrotating relative to the guide ring. A clutch ring applies a frictionalforce to the guide ring to resist rotation until the segments engage thetendon threads.

The bottom end of the tendon is attached to a foundation—any structurethat is anchored to the sea floor—and is preferably an existingfoundation, although foundation anchor points can be added if needed.Tendons can be connected to a foundation through any suitable anchorconnector, and many are available in the art. Anchor connectors aredescribed, for example, in U.S. Pat. No. 4,498,814, U.S. Pat. No.4,611,953, EP0319419, U.S. Pat. No. 4,943,188, U.S. Pat. No. 4,374,630,U.S. Pat. No. 4,907,914, U.S. Pat. No. 5,004,272, and U.S. Pat. No.6,568,875. OTC: 4947-MS describes the connectors used in Conoco's Huttonfield TLP.

ConocoPhillips has also developed tendon anchoring devices. U.S. Pat.No. 4,844,659 describes an apparatus for attaching a floating tensionleg platform to an anchoring base template on the subsea floor includesan external mooring porch for each tendon, the porches being mounted onthe outside surfaces of the platform's columns. The number of porchesmay exceed the number of columns by a factor of at least two or three.

U.S. Pat. No. 5,324,141, also by ConocoPhillips, describes another suchbottom connector, wherein the apparatus includes a tendon with anenlarged connector on one end, the connector being formed by afrustoconical bearing surface extending away from the end to which it isconnected. A connector shroud partially surrounds the frustoconicalsurface and is connected to it by an elastomeric bearing. An inwardlysloping surface engages a supplementarily sloped surface formed on theattachment receptacle load ring to increase hoop strength. A side-entrybottom receptacle is also disclosed.

The tendon attachment point(s) can be modified for use with the variousoffshore units. In one embodiment, the tendons can be attached to apiled foundation or sockets (depending on the type of offshore unit). Inanother embodiment, a single deep-set, large diameter pile can serve asthe attachment point for multiple tendons.

U.S. Pat. No. 6,036,404 describes a foundation system for tension legplatforms without use of foundation templates, wherein each tendon isdirectly connected to a socket inside the pile, said piles beingpositioned for driving purpose by means of a pile-driving template whichis employed as a spacing device. The pile-driving template is positionedwith the aid of pins that slot into guides built into the well template.After the groups of piles needed to anchor a corner of the platform havebeen driven in, the pile-driving template is withdrawn and repositionedso as to enable the piles for the other group of legs to be driven; thisprocess continues until all of the pile-driving is finished.

Mooring systems are described in US2011206466, which includes tendonssupported at tendon porches directly at the four column outboard lowercorners, without additional radially-extending tendon supportstructures.

A hydraulic jack assembly or other tensioning device can be used totension the tendons. Usually, the tendons are configured in such a wayas to form a straight line alongside (or inside) the legs, but can alsobe at an angle, e.g., about 45° from horizontal, although this is lesslikely.

The tendons can be tensioned in place using a hydraulic jack assembly,winch, or by opposing forces (buoyance v. hold down force or stretchingas the jack-up is raised). In other embodiments, the tendons can betensioned as described in US2006210362, U.S. Pat. No. 5,551,802, or U.S.Pat. No. 7,044,685. For example, the tension can first be applied to apull-down line (such as a steel chain) by a winch, and later transferthe tension to the tendon by engaging the tendon to a fixed sleeve andrelease the pull-down line.

Self tensioning legs are also available, and may be particularlypreferred as easy to install. See e.g., US2010232886. This patentdescribes a self-tensioning tendon (of the pipe type) having ahydraulically controlled Length Adjustment Joint (LAJ) containing anintegral cylinder with external threads and a piston rod. A hydraulicsource actuates the integral cylinder to pull the TLP down to the targetdraft position, significantly reducing the time needed to ballast thehull of the TLP with millions of gallons of water. A Top TendonConnector (TTC) ratchets down along the cylinder to lock the TLP at thefinal draft position.

US20110052327 describes a floating hull and a method of connection ofsuch a hull to tension legs, which avoids large bending forces on theconnector and which can be rapidly anchored to the sea bed viapre-installed tension legs without the need for installation supportvessels. The attachment means comprise a guide member for lowering atensioning member section by a predetermined length, which tensioningmember section at a free end is provided with a complementary connectorfor attaching to the connector on the upper end of the tensioningmember, the tensioning member section comprising at an upper end astopper for engaging with the base and for fixing the upper end in avertical direction, the floating construction comprising a pullingdevice attached to the tensioning member section, for lowering theuncoupled tensioning member section along the guide member.

Tension legs can be any known in the art, and in the past have typicallybeen tubular pipes, joined end to end. As an example, the world's firstcommercial tension leg platform by Conoco at Hutton field utilizes aplurality of tubular joints thirty feet in length having a ten-inchouter diameter and a three inch longitudinal bore. However, modernmethods also use tension leg cables, which can be easier to transportand deploy due to their flexibility. Pressurized gas filled tendons arealso known (U.S. Pat. No. 4,521,135), as are pressurized liquid filledtendons (U.S. Pat. No. 4,664,554).

Corrosion resistant cables are taught in U.S. Pat. No. 4,285,615 byConoco, and such may be particularly preferred. These multi-strandcables have voids between adjacent strands, a fluid tight covering, anda fluid, such as nitrogen or argon, filling all spaces between the two.

The first installed TLP, on Conoco's Hutton field, had 10.25 inch (0.26meter) OD, thick-walled tubular tendons, but in general, industry istending toward larger diameters with D/t ratios of 20 to 30. Thus, thetendons may preferably be large diameter and thin walled, typically 48inch (1.22 meter) outside diameter (OD) with a diameter to thickness(D/t) ratio greater than 30. See OST: 5937-MS. Compared with heavy-wall,small diameter tendons, the advantages of large D/t tendons are thatthey: (1) have greater buoyancy, which reduces the weight; (2) permit aleak-before-break inspection philosophy; (3) can be fabricated by moreefficient pipe rolling methods, thereby reducing overall tendon cost,and (4) may have greater steel area in each tendon, thereby reducing thenumber of tendons and tendon support hardware.

One cable that may be preferred is a 122 mm diameter cable with abreaking strength elf 8000 KN. The core is composed of layers of roundwires 7 mm in diameter, made of drawn high carbon steel. An amorphouspolypropylene filling medium is pressure applied during spinning. Onthis center two layers of Z profile wires are stranded 7 mm thick. Theyare made of cold drawn and rolled high carbon steel. The peripherallayer is composed of pure zinc Z wires and form a continuous concentricanode. An outer sheath of polyurethane is then pressure-extruded ontothis full locked type assembly. See e.g., OTC: 4052-MS. This cable isallegedly easy to handle because of its plastic jacket and its internalwire arrangement; and well protected against water penetration becauseof the inside filling material, the presence of three layers of Z wires,and the presence of the external sheath.

Where the tendon needs to be compiled from multiple lengths, connectorswill be needed. Such connectors are well known in the art, and includethreaded joints for tubular legs, releasable joints (U.S. Pat. No.4,869,615), and the like.

Length-adjustment unit(s) may also be provided in a tendon to eitherincrease or reduce the tension for different purposes. For example, whenice or high seas nudges the hull upward to an extent that the tendon orthe hull may be damaged, extending the length of the tendon canalleviate the stress.

For jack-up rigs in particular, the tendons are expected to increase theflexibility of ice worthy JUs for Arctic operations, and increase theability of rigs to operate in areas with high wind, wave and ice loads.

The phrase “horizontal environmental loads” or “horizontal loads” areused interchangeably and mean any environmental or climatic force (wind,waves, ice, current, etc.), generally perpendicular to gravity, whichacts on a structure causing it to drift, slide or topple. As usedherein, this also applies to horizontal loads that can cause structuraldamage to supports, i.e. ice hitting legs on a jack-up.

The phrases “offshore unit”, “drilling unit”, and “unit” are usedinterchangeably and mean any unit used in water for oil and gasdrilling, production, or completion. This includes both open and closedwaters.

The phrase “legs” as used herein mean any solid structure (as opposed totendons, which are flexible) used to support a sea floor anchoredoffshore oil and gas unit. These can include, but is not limited to,open-truss legs, columnar legs, ballasts, and other gravity-basedsupport structures.

By “tendon” what is meant is a flexible tube, bar, wire or cables. Cablestructure consists of multiple wires bonded, twisted or braided togetherto form a single assembly, but preferred cables are 7×19 stainless steelwire rope, 3×19 galvanized cable, spiral strand cable, and the like, andmay be wrapped or otherwise coated for increased corrosion resistance.For example, mooring cables are often jacketed in polypropylene. In thisapplication heavy tube would be the most likely tendon as it could carryheavier loads that a cable and stretch less and would be easier totension.

By “tensioned” tendon what is meant is that force is applied to tendon,tightening it in place, and not applied by an external, variable forceas would be the case with e.g., a loose mooring line being variablytightened and loosened by wave action. The tensioning force will tend topull the hull down, whereas the jack-up legs will oppose that force.

By “flexible” in relationship to tendons what is meant is that thetendons are too flexible to support the weight of the rig, as opposed tothe legs, which are constructed to support the weight of the rig. Thus,a tubular tension leg, even though not flexible over a short length,will be too flexible over the entire length to support a jack-up unitthat is no longer buoyant.

Preferred materials are corrosion and salt resistant, and includestainless steel, carbon fiber reinforced steel, and properly protectedsteels. Deep sea mooring cables in synthetic materials are also known,but in certain situations may be less preferred as subject to biologicalattack.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims or the specification means one or more thanone, unless the context dictates otherwise.

The term “about” means the stated value plus or minus the margin oferror of measurement or plus or minus 10% if no method of measurement isindicated.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or if thealternatives are mutually exclusive.

The terms “comprise”, “have”, “include” and “contain” (and theirvariants) are open-ended linking verbs and allow the addition of otherelements when used in a claim.

The phrase “consisting of” is closed, and excludes all additionalelements.

The phrase “consisting essentially of” excludes additional materialelements, but allows the inclusions of non-material elements that do notsubstantially change the nature of the invention.

The following abbreviations are used herein:

CPM Conical piled monopods GBS Gravity based structures IWJU Ice-worthyJack-up JU Jack-up TLP Tension leg platform

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A. Common offshore drilling units and preferred use depth.

FIG. 1B. A perspective view of a prior art tension leg platform (TLP),showing vertical columns that are connected together by a ring-shapedpontoon, which is located inboard of the columns.

FIG. 1C. is a plan view in cross section taken along lines 4-4 of FIG.1B of the hull (columns and pontoons) of the tension leg platform.

FIG. 2A illustrates a jack-up rig on location before the legs arelowered.

FIG. 2B illustrates a jack-up rig attached to spud cans resting on thepile. Tendons have been lowered and attached to the foundation.

FIG. 2C illustrates a jack-up rig on location and hull or platformraised above the water level.

FIG. 3 illustrates an ice-worthy jack-up (IWJU) in an “ice defensiveconfiguration” with tendons attached to the pilings.

FIG. 4 illustrates the hydraulic jack assembly.

FIG. 5A-C. One tensioning method.

DESCRIPTION OF EMBODIMENTS

The present disclosure provides a novel method and devices forpreventing an offshore unit from overturning due to horizontalenvironmental and climatic loads using tendons capable of beingtensioned. This support method can easily be modified for use withjack-ups (JUs), ice-worthy jack-ups (IWJUs, e.g., a JU having an icehull), conical piled monopods (CPMs), and gravity based structures(GBS).

One of the novel aspects of this method includes the use of tendons,capable of being tensioned, attached to the foundation and the offshoreunit. Essentially, the tendons are attached adjacent to the legs and areattached to the sea floor foundation. The tendons are then ‘tensioned’by the opposing forces (i.e. buoyant or jacked-up platform andimmobilized foundation) present. Thus, the novel units have both legsand tendons and thus greatly improved stability.

The invention may include one or more of the following embodiments:

A method for reinforcing an offshore oil and gas unit that is anchoredto the sea floor comprising:

a) providing an offshore oil and gas unit (OOGU) that is that isanchored to the sea floor with at least one leg;

b) extending a tendon from the OOGU to a foundation on the sea floor;and

c) tensioning said tendon sufficiently to reinforce the OOGU.

The offshore drilling unit can be a jack-up, an ice-worthy jack-up, aconical piled monopod, or any gravity based structure, or any unit alsohaving solid legs.

Another embodiment is a jack-up rig for offshore drilling forhydrocarbons comprising:

a) a flotation hull having a relatively flat deck at the upper surfacethereof;

b) at least three legs that are positioned within a perimeter of theflotation hull wherein the legs are arranged to be i) lifted up off theseafloor so that the Jack-up may be towed through shallow water and ii)extend to the sea floor and extend further to lift the hull partially orfully out of the water;

c) a jack-up device associated with each leg to raise and lower saidleg; and

d) a plurality of tendons having a top end attached to the jack-up unitand a bottom end configured to be attached to a sea floor foundation.

The tension legs or tendons can have a top end is attached to the hullor to one of said legs.

The foundation can be any suitable foundation, and preferably usesexisting foundation structures, such as a piled foundation. One or morethan one tendon is attached to said foundation. The tendons can betubular tendons, or cable tendons, but preferably include a selftensioning tendon.

In other embodiments, an improved jack-up rig is provided, jack-up rigsincluding a buoyant hull, legs traversing said hull, and a jackup devicefor each leg such that the buoyant hull can be raised out of the waterfor use, the improvement comprising tension legs having a top endattached to said jack-up rig and a bottom end attachable to a foundationon the sea floor.

In yet another improved jack-up rig, the improvement includes tensionlegs having a top end attached to said jack-up rig and a bottom endattached to a foundation on the sea floor, said tension legs furtherhaving a self-tensioning unit for adjusting the length and thus tensionof each tension leg.

Some embodiments of the disclosure are exemplified with respect to FIGS.2-5.

Although the systems, processes, and alternative designs included hereinhave been described in detail, these figures are exemplary only. Itshould be understood that various changes, substitutions, andalterations could be made without departing from the spirit and scope ofthe invention. Those skilled in the art will be able to identify otherways to practice the invention that are not explicitly described herein.The following examples are intended to be illustrative only, and notunduly limit the scope of the appended claims.

Prior Art Tension Leg Platform

A prior art TLP (from US2011206466) is shown in FIGS. 1B and 1C. FIGS.1B-C show a tension leg platform 10 for use in offshore applications.The platform 10 has a hull 11 including vertical support columns 12 anda central pontoon structure 14 disposed inboard of the columns at alower end thereof. TLP 10 includes a deck structure 13 supported by theupper ends of the columns 12.

The interior of both the columns 12 and the pontoon structure 14 ispreferably subdivided by structural bulkheads (not illustrated) tostrengthen the structure, to provide enclosed spaces for locating andstoring various equipment (e.g., anchors, chains, propulsion mechanisms,etc.), and to provide a plurality of separate tanks for purposes ofballasting the vessel and storing various fluids, equipment, and othermaterials which may be required or desired during drilling or productionby the well.

TLP 10 is anchored by a plurality of vertical or near vertical mooringtendons 17 that are connected to tendon porches 18 on the lower end ofthe outboard face of the columns 12. Each column 12 is designed to matewith at least one, but usually two or more tendons 17. The tendonporches are positioned near the keel elevation and contain connectionsleeves (not illustrated) to receive the upper tips of the tendons 17and clamp thereto. The connection sleeves may be ring-shaped, requiringvertical entry of the tendons 17, or they may be slotted to allow sideentry of the tendons 17.

Various types of risers 19 can be supported by the hull 11, includingnear-vertical top tensioned risers (TTR), flexible risers, or steelcatenary risers (SCR). The flexible risers or steel catenary risers(SCRs) can be supported on the inboard or the outboard side of thecentral pontoon structure 14, and extended to the deck 13 by either asingle span spool piece or by piping supported on the hull. The toptensioned risers (TTRs) can be supported on the deck 13, and can also besupported laterally at the pontoon elevation by riser keel joints (notillustrated).

Although any suitable shape may be used, the central pontoon structure14 is octagonal-shaped, having four orthogonally-oriented side segments14 a intervaled with four diagonally-oriented corner segments 14 b thatare connected to the pontoon structure 14 to form a unitized structurecentered about the platform central vertical axis C. In the embodimentshown in FIGS. 1B-C, the central pontoon structure 14 includes a centralmoonpool opening 14 c, which is illustrated as an octagonal opening butmay have any other suitable shape. Side and corner segments 14 a, 14 bare each preferably characterized by generally rectangular transversecross section surrounding a central horizontal axis or horizontalcenterline HC.

Each of the vertical columns 12 has a lower end 12 a and an upper end 12b. The columns 12 preferably have a quadrilateral transverse(horizontal) cross-section, which may be a generally rectangular ortrapezoidal-shaped configuration. FIGS. 1B-C show columns 12 asrectangular, having a transverse cross-sectional shape with a major axisA1 oriented radially outward from a center point C of the hull 11.

Specifically, columns 12 define a rectangular transverse cross sectionformed of two parallel spaced wider lateral side walls 12 c connected tonarrower inner and outer side walls, 12 d, 12 e, respectively. Thus,each vertical support column 12 defines a major axis A1 extendingbetween the inboard and outboard side walls, 12 d, 12 e, and a minoraxis A2 extending between the two lateral side walls 12 c. Each verticalsupport column 12 defines a vertical longitudinal axis or verticalcenterline VC at the intersection of major axis A1 and minor axis A2.The major axis A1 of each of the vertical support columns 12 ispreferably oriented radially outward from the center C of the platform.A lower portion of inboard side wall 12 d of each vertical supportcolumn 12 abuts and is joined to a respective diagonal corner segment 14b of the pontoon structure 14.

Vertical support columns 12 are disposed substantially outboard of thecentral pontoon structure 14. The vertical axis VC of each column 12 isdisposed a distance D1 outwardly from the outer periphery of cornersegment 14 b of the pontoon structure 14 and a distance D2 outwardlyfrom the central horizontal axis or horizontal centerline HC extendingthrough the pontoon corner segment 14 b. Thus, with this hullconfiguration the central pontoon structure 14 is positioned inboard ofthe vertical support columns 12, such that a line S defined between thevertical centerlines VC of two adjacent columns 12 lies outside thehorizontal centerline HC of the pontoon side segments and, morepreferably, outside the outer periphery of the pontoon structure 14.This design feature differs from previous tension leg platform designs,which typically have individual pontoons centered between the columns,with the vertical centerlines of the support columns intersecting thehorizontal centerlines of the adjacent pontoons.

One or more of the TLP features (especially tendon legs, connectors,tensioning systems and the like) may be incorporated into the jack-uprig and other structures claimed herein.

Installing Jack-Up with Tension Tendons

FIG. 2A shows a jack-up rig 210 at a pre-determined location. Thegenerally planar deck 221, where the drilling equipment 230 is located,of the floating hull 220 rest above the water line 212. The feet (e.g.,the bottom of the legs) 226 have been connected to spud cans 228 beforebeing towed to the pre-determined location. Once positioned above thepilings 236 located on the sea floor 215, the rig legs 225 will belowered through openings 227 in the hull. The spud cans 228 will beconnected to the piled socket 235.

In FIG. 2B, the legs 225 have been lowered and the spud cans 228 havebeen connected directly to the piled sockets 235 anchored to the seafloor 215. The piled sockets 235, after having been precisely positionedon the sea floor 215, are secured to the seafloor by pilings 236 thatkeep the sockets 235 from lifting up or settling lower into the seafloor. Once the legs 225 are connected to the foundation, the tendons237 can be lowered and attached to the piled sockets 235 or anotherfoundation means. Once the tendons are in place, the hull or platform ofthe jack-up rig can be raised.

In an alternative configurations (not shown), the tendons can beattached to a single pile, resulting in tendons not running parallel tothe legs. In yet alternative configurations not shown, the tendons canbe attached to a multiple piles outside the perimeter of the rig,resulting in tendons angled away from the vertical.

In FIG. 2C, the deck 221 has been raised above the water line 212. Thetendons 237 are stretched between the top of the legs 225 and thesockets 235. Generally, the tendons will experience some tensionbecause, much like rubber bands, they will want to return to theiroriginal length but cannot due to being anchored by the foundation. Ahydraulic system (not shown) can be used to increase the tension of thetendons 237. Increase in tension is especially necessary for harshenvironmental conditions found in places such as the Arctic. Moving iceand land fast ice can displace an ice worthy jack-up even when it is inan ice defensive position (see FIG. 3).

The jack up rigs can have one or more legs, and more commonly three orfour legs, however, for simplicity, only two legs are shown here.Furthermore, while open-truss legs are shown in the figures, columnarlegs can also be used because the tendons can be outside the legstructure. Generally, only one tendon is used per leg, however, instrenuous environments, such as the Beaufort Sea, more tendons can beused.

Spud cans are typically used on independent-legged jack-up rigs and aredesigned to spread the load so that the unit does not sink too deeplyinto the sea floor. Other types of supports for the rig legs includemat-supported which distribute the weight of the unit across the ocean'sbottom, much like a snow shoe. Any type of support system can be usedwith the present devices and methods.

When support systems are not used, the jack-ups are usually ‘preloaded’into the sea floor to simulate the maximum expected leg loads. Thisoccurs by using the weight of the platform, hull and ballast to drivethe legs into the sea floor. In theory, the legs will not penetrate anyfurther into the sea floor during operations. For these types of setups, a single pile can be installed and used as the foundationattachment point.

Tensioning Tendons

In the current devices and methods, the tendons can be tensioned using ahydraulic jack assembly to provide additional hold down forces in thefoundation. The operation and utilization of this tension control systembegins after the tendons and legs have been attached to the foundationor support system, and generally after the j

FIG. 4 illustrates an exemplary hydraulic jack assembly. This systemincludes one or more hydraulic jacks 467. Each jack has a cylinder 463coupled through a load block 462 to a tension tendon 446, and a floatingpiston 466 cooperating with the load block plug (not shown) coupled tothe deck. A collar means 468 is disposed about the tendon and is adaptedfor movement in cooperation with the anchoring plate (not shown) on theoffshore unit to grip the tendon. A flange 464 helps to hold the loadblock to the offshore unit. The floating piston 466 is contained insidethe cylinder 463 and is moveable. The bore of the load block is alignedwith the core of the cylinder such that the load block plug can slidethrough the bore of the load block and abut the floating piston.

An accumulator 460 supplies hydraulic fluid 61, under pressure, to thejacks to compensatingly adjust the relative position of the floatingpiston 66 and cylinder 67 to selectively maintain the tension loading ofthe tendon as the rig hull is ‘jacked up.’

Alternative Tensioning of Tendons

In the alternative, tension can be applied to the tendons on site.Referring to FIG. 5A, a winch 444 located on the deck 448 can be used towind a pull-down line 446, which may be a steel chain. At the lower endof the pull-down line there is provided an engaging means 532A, whichwill be coupled with the tip 532B of a tendon 512, as shown in FIG. 5B.The hull 516 preferably also has corresponding number of sleeves 522 tosecurely lock the tendons in position.

As shown in FIG. 5C, the pull-down line 546 is first guided inside thesleeve 522 to further lower into the water, where the engaging means532A will couple with a corresponding tendon 512 at its tip 532B. Afterthe engaging means of the pull-down line 546 is coupled with the tendon512, the winch 544 then applies tension to the pull-down line by reelingup the pull-down line. When the tendon 512 is in place within the sleeve522, as shown in FIG. 5D, the sleeve can then clamp the tendon 512 orequivalently lock the tendon 512 in position. At this point of time,relieving the tension by the winch 544 to slack the pull-down line 546will transfer the tension to the tendon 512. Additional tension can beapplied if the tendons stretch, which may be of significance if thecable tendons are used that were not pre-stretched before deployment.

Additionally, a length adjustment unit can be provided for each tendon.The length of the tendon can be adjusted by the length adjustment unit.Such adjustment may be useful if relieving a certain amount of tensionis beneficial, for example if the ice load pushes hull upward to theextent of damaging the tendon, or if slack is needed for other reasons.

The following references are incorporated by reference in theirentirety.

-   -   EP0319419    -   EP0966396    -   OTC: 5937-MS    -   OTC: 4052-MS    -   OTC: 4947-MS    -   US2006210362    -   US2009290939    -   US20100232886    -   US20110052327    -   US20110206466    -   US20120125688    -   U.S. Pat. No. 4,265,568    -   U.S. Pat. No. 4,285,615    -   U.S. Pat. No. 4,374,630    -   U.S. Pat. No. 4,498,814    -   U.S. Pat. No. 4,521,135    -   U.S. Pat. No. 4,604,001    -   U.S. Pat. No. 4,611,953    -   U.S. Pat. No. 4,664,554    -   U.S. Pat. No. 4,742,993    -   U.S. Pat. No. 4,844,659    -   U.S. Pat. No. 4,869,615    -   U.S. Pat. No. 4,871,282    -   U.S. Pat. No. 4,907,914    -   U.S. Pat. No. 4,907,914    -   U.S. Pat. No. 4,943,188    -   U.S. Pat. No. 5,004,272    -   U.S. Pat. No. 5,020,942    -   U.S. Pat. No. 5,324,141    -   U.S. Pat. No. 5,441,008    -   U.S. Pat. No. 5,551,802    -   U.S. Pat. No. 6,036,404    -   U.S. Pat. No. 6,568,875    -   U.S. Pat. No. 7,044,685

What is claimed is:
 1. A method for reinforcing an offshore oil and gasunit that is anchored to the sea floor comprising: a) providing anoffshore oil and gas unit (OOGU) that is that is anchored to the seafloor with at least one leg; b) extending a tendon from the OOGU to afoundation on the sea floor; and c) tensioning said tendon sufficientlyto reinforce the OOGU.
 2. The method of claim 1, wherein said offshoredrilling unit is a jack-up, an ice-worthy jack-up, a conical piledmonopod, or a gravity based structure.
 3. A jack-up rig for offshoredrilling for hydrocarbons comprising: a) a flotation hull having arelatively flat deck at the upper surface thereof; b) at least threelegs that are positioned within a perimeter of the flotation hullwherein the legs are arranged to be i) lifted up off the seafloor sothat the jack-up may be towed through shallow water and ii) extend tothe sea floor and extend further to lift the hull partially or fully outof the water; c) a jack-up device associated with each leg to raise andlower said leg; and d) a plurality of tendons having a top end attachedto the jack-up unit and a bottom end configured to be attached to a seafloor foundation.
 4. A jack-up rig as in claim 3), wherein said top endis attached to said hull.
 5. A jack-up rig as in claim 3), wherein saidtop end is attached to one of said legs.
 6. A jack-up rig as in claim3), wherein said foundation is a piled foundation.
 7. A jack-up rig asin claim 4), wherein more than one tendon is attached to said piledfoundation.
 8. A jack-up rig as in claim 3), wherein said tendons aretubular tendons.
 9. A jack-up rig as in claim 3), wherein said tendonsare cable tendons.
 10. A jack-up rig as in claim 3), wherein saidtendons comprise a self tensioning tendon.
 11. An improved jack-up rig,jack-up rigs including a buoyant hull, legs traversing said hull, and ajack for each leg such that the buoyant hull can be raised out of thewater for use, the improvement comprising tension legs having a top endattached to said jack-up rig and a bottom end attachable to a foundationon the sea floor.
 12. An improved-up rig that includes a buoyant hull,legs traversing each hull and a jack-up device for each leg such thatthe buoyant hull can be raised out of the water for use, the improvementcomprising tension legs having a top end attached to said jack up rigand a bottom end attached to a foundation on the sea floor, said tensionlegs further having a self-tensioning unit for adjusting the length andthus tension of each tension leg.